title: Structural insights into a unique cellulase fold and mechanism of cellulose hydrolysis.
abstract: Clostridium thermocellum is a well-characterized cellulose-degrading microorganism. The genome sequence of C. thermocellum encodes a number of proteins that contain type I dockerin domains, which implies that they are components of the cellulose-degrading apparatus, but display no significant sequence similarity to known plant cell wall-degrading enzymes. Here, we report the biochemical properties and crystal structure of one of these proteins, designated CtCel124. The protein was shown to be an endo-acting cellulase that displays a single displacement mechanism and acts in synergy with Cel48S, the major cellulosomal exo-cellulase. The crystal structure of CtCel124 in complex with two cellotriose molecules, determined to 1.5 Å, displays a superhelical fold in which a constellation of α-helices encircle a central helix that houses the catalytic apparatus. The catalytic acid, Glu96, is located at the C-terminus of the central helix, but there is no candidate catalytic base. The substrate-binding cleft can be divided into two discrete topographical domains in which the bound cellotriose molecules display twisted and linear conformations, respectively, suggesting that the enzyme may target the interface between crystalline and disordered regions of cellulose.
authors.affiliation: Centro de Investigação Interdisciplinar em Sanidade Animal, Faculdade de Medicina Veterinária, Pólo Universitário do Alto da Ajuda, Avenida da Universidade Técnica, 1300-477 Lisbon, Portugal.
authors.forename: Joana L A
authors.initials: JL
authors.lastname: Brás
authors.forename: Alan
authors.initials: A
authors.lastname: Cartmell
authors.forename: Ana Luísa M
authors.initials: AL
authors.lastname: Carvalho
authors.forename: Genny
authors.initials: G
authors.lastname: Verzé
authors.forename: Edward A
authors.initials: EA
authors.lastname: Bayer
authors.forename: Yael
authors.initials: Y
authors.lastname: Vazana
authors.forename: Márcia A S
authors.initials: MA
authors.lastname: Correia
authors.forename: José A M
authors.initials: JA
authors.lastname: Prates
authors.forename: Supriya
authors.initials: S
authors.lastname: Ratnaparkhe
authors.forename: Alisdair B
authors.initials: AB
authors.lastname: Boraston
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Romão
authors.forename: Carlos M G A
authors.initials: CM
authors.lastname: Fontes
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Gilbert
mesh_terms: D002240:Carbohydrate Sequence; D020134:Catalytic Domain; D002480:Cellulase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002482:Cellulose / Q000378:metabolism; D048013:Acetivibrio thermocellus / Q000201:enzymology / Q000235:genetics; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D009844:Oligosaccharides; D002482:Cellulose; D002480:Cellulase